α-synuclein (α-Syn) is a main constituent protein of an inclusion body found in Parkinson's disease (PD) and dementia with Lewy bodies (DLB). The protein is thought to acquire cytotoxicity by aggregation to cause disease. α-synuclein oligomer, which is an aggregation intermediate of α-Syn, is reported to exhibit higher cytotoxicity than amyloid fibril as a final product. It is thus suggested that the α-synuclein oligomer is likely to be a main body of toxicity causing PD and DLB.
In addition, not limited to α-synuclein, accumulations of amyloid proteins such as 13 amyloid peptide and tau protein, prion protein, polyglutamine peptide, and huntingtin, respectively, are significantly associated with the occurrence of diseases such as Alzheimer's disease, prion disease, polyglutamine disease, and Huntington's disease, respectively. These proteins are also ever present in soluble forms in the body and change their conformations due to some stimuli to form oligomers, then resulting in amyloid fibril formation in insoluble forms.
Regarding diseases caused by depositions or the like of these proteins, specific detection of amyloid protein oligomer is needed to elucidate the pathogenesis of the diseases. Additionally, knowing the localization of the oligomers in tissues is useful for the prevention and diagnosis of such diseases.
Accordingly, various techniques have been developed to specifically detect oligomeric amyloid proteins. For example, Japanese Unexamined Patent Application Publication (JP-A) Nos. 2010-531992 and 2010-530227 disclose anti-amyloid β peptide antibodies. In addition, JP-A Nos. 2010-502938 and 2010-537962 disclose anti-amyloid β protein monoclonal antibodies.
Meanwhile, an aptamer is known as a polynucleotide molecule that specifically binds to a certain molecule. Aptamers can be totally chemically synthesized using a commercially available nucleic acid synthesizer and thus are advantageous in terms of obtaining at much lower cost than specific antibodies and allowing the designing of structural changes due to easy modification. Furthermore, unlike peptides, aptamers return to their original state even when denatured at high temperature and therefore are highly stable. Aptamers form a specific three-dimensional structure to have a molecular recognition ability selective to a target molecule. As the three-dimensional structure of aptamers, various shapes have been reported, such as those of hairpin, pseudoknot, bulge, and G-quartet.
As an aptamer for detecting α-synuclein, M5-15 is known, which has plural hairpin structures (see Biotechnol. Lett., (2010) vol. 32, pp. 643-648). The α-synuclein-binding aptamer M5-15 is described to recognize not only synuclein oligomer but also α-synuclein monomer.